A pipe flange with a heavy welding neck which is hot formed from a circular section of steel plate having a small co-axial hole in the center.

The steel around the hole is shear-formed downward and outward by a tapered die plug which is forced through the small hole, forming an integral neck on the flange. A supporting ring die is radiused at its inner corner to form a suitable fillet radius at the junction of the neck with the flange proper. A suitable preforming or post-forming operation may be required to compensate for any deflection in the workpiece out of planar configuration.

The method of forming a weld neck flange comprising the steps of:

1.forming a small, generally coaxial circular opening in a generally circular workpiece of thick mill-rolled steel plate material;

2.heating said workpiece to glowing;

3.placing said workpiece, with one flat annular surface up, on an open female die;

4.said female die having a top annular support surface and a generally cylindrical cavity;

5.then, pressing a blunt convex circular member against the top surface of said workpiece while supported on said annular support surface so that said top surface of the workpiece is formed concave and the opposite surface is formed convex;

6.inverting said workpiece so that said convex opposite surface is up;

forcing through said heated workpiece a male die having an entry nose that enters freely through 7.said circular opening and a profile gradually increasing in diameter to engage around the edges thereof and shear-form the steel closely surrounding said opening downward below the bottom surface of said workpiece;

8.said profile of said male merging with a trailing cylindrical portion thereof of substantially the internal diameter to be formed.

Threaded flanges which also named screwed flanges look very much like a Slip On flange in some ways. The main difference is the threaded flange was bored out initially to match a specific pipe inside diameter. The backside of this center opening is then threaded with the proper sized tapered pipe thread.

Threaded Flanges are used for special circumstances with their main advantage being that they can be attached to the pipe without welding.

 Sometimes a seal weld is also used in conjunction with the threaded connection. 

This special flange is generally used for high pressure, yet smaller diameter applications. Obviously, the larger the flange is, the harder the ability to machine a thread would be. Threaded flanges are also not a good choice for a system that has a high level of torque. 

Key Features:

Can be attached to the pipe without welding (sometimes an additional seal weld is used as well)

Ideal for higher pressure and temperature applications for small pipe sizes

Not recommended for larger loads, especially when subjected to higher torque

The slip-on flange may be attached to the end of a piece of pipe or to one or more ends of a pipefitting. The slip-on flange is positioned so the inserted end of the pipe or fitting is set back or short of the flange face by the thickness of the pipe wall plus 1/8 of an inch. This allows for a fillet weld inside the SO flange equal to the thickness of the pipe without doing any damage to the flange face. The back or outside of the flange is also welded with a fillet weld.

A variation of the Slip-On flange also exists. This is the Slip-On Reducing Flange. This is simply a larger (say a 14") Slip-On flange blank that, instead of the Center (pipe) hole being cut out (or drilled out) for 14" pipe it is cut out for a 6" pipe. The SO Reducing flange is basically used for reducing the line size where space limitations will not allow the length of a weld neck flange and reducer combination. The use of the Slip-On Reducing Flange should only be used where the flow direction is from the smaller size into the larger size.

There are many flange manufactures and different flange standards to be found worldwide. To allow easy functionality and inter-changeability, these are designed to have standardised dimensions. Common world standards include ASA/ANSI (USA), PN/DIN (European), BS10(British/Australian), and JIS/KIS (Japanese/Korean).

The flange faces are made to standardized dimensions and are typically "flat face", "raised face", "tongue and groove", or "ring joint" styles, although other obscure styles are possible.

Flange designs are available as "welding neck flange", "slip-on flange", "boss", "lap joint", "socket weld flange", "threaded", and also "blind flange".

ANSI designations such as ANSI 150, ANSI 300 and so on are often followed by a # (pound sign). The ANSI number does not directly relate to a pressure rating, but to a class of flange. For example, the hash (#) or 'pound' reference; e.g. 300 pound, can be misleading in that an ANSI 300 flange is actually rated for a test pressure of 740 psi (~5100 kPa), and only within a certain working temperature range.

In most cases these are not interchangeable (eg an ANSI flange will not mate against a JIS flange). Further many of the flanges in each standard are divided into "pressure classes", allowing flanges to be capable of taking different pressure ratings. Again these are not generally interchangeable (eg an ANSI 150 will not mate with an ANSI 300). These "pressure classes" also have differing pressure and temperature ratings for different materials. "Pressure Classes" of piping are usually developed for a process plant or power generating station; these "pressure classes" may be unique to the specific corporation, Engineering Procurement and Construction (EPC) contractor, or the process plant owner.

There are many kinds of flanges, such as special flat flanges ,stainless steel flat  flange, plate flange and so on.

Although flange generally refers to the actual raised rim or lip of a fitting, many flanged plumbing fittings are themselves known as 'flanges'. Common flanges used in plumbing are the Surrey flange or Danzey flange, York flange, Sussex flange and Essex flange. Surrey and York flanges fit to the top of the hot water tank allowing all the water to be taken without disturbance to the tank. They are often used to ensure an even flow of water to showers. An Essex flange requires a hole to be drilled in the side of the tank.

There is also a Warix flange which is the same as a York flange but the shower output is on the top of the flange and the vent on the side. The York and Warix flange have female adapters so that they fit onto a male tank, whereas the Surrey flange connects to a female tank.